docs: def metadata(self, filename):
          '''
          Get some metadata for a given file.
  
          Can vary from a backend to another but some are always present:
          - `filename`: the base filename (without the path/prefix)
          - `url`: the file public URL
          - `checksum`: a checksum expressed in the form `algo:hash`
          - 'mime': the mime type
          - `modified`: the last modification date
          '''
          metadata = self.backend.metadata(filename)
          metadata['filename'] = os.path.basename(filename)
          metadata['url'] = self.url(filename, external=True)
          return metadata